1. Field of the Invention
The present invention relates to the field of semiconductor circuit manufacturing, and more specifically to the forming of buried layers between a substrate and an epitaxial layer.
2. Discussion of the Related Art
Such a structure is shown as an example in FIG. 1. The substrate is designated by reference 1, the epitaxial layer is designated by reference 2, and buried layers are designated by references 3, 4, and 5. If layers 3 and 5 are of the same type, they will have at the end of the process the same extension in height. If layer 4 is of a different type, it may have a height different from that of layers 3 and 5.
Conventionally, these buried layers are formed by implantation of selected dopants in the substrate before the epitaxial layer is formed. The implantation step is necessarily followed by a brief restructuring anneal to eliminate structural defects induced in the substrate by the implantation. Indeed, if such structural defects were left to remain, the epitaxial layer could exhibit defects. However, usually, a stronger anneal than the anneal necessary for only the substrate restructuring is performed. After the epitaxy step, due to various anneal steps necessary to a satisfactory activation of the buried layers and inevitably resulting from the integrated circuit manufacturing, the buried layers have diffused vertically and laterally with respect to the initial location of the implantation. Thus, to take account of the lateral diffusion, a minimum distance has to be provided between two masks of definition of implanted regions intended for forming buried layers of opposite types. Otherwise, neighboring buried layers would join, which would bring about various disadvantages well known by those skilled in the art, especially as concerns the breakdown voltage of the device and the creation of stray capacitances, not to mention the fact that, due to the mixing between implanted areas of distinct types, the doping characteristics initially aimed at are not obtained.
Further, intrinsically, the fact of having to provide diffusions after implantation of the buried layers results in that it is not possible to go under a minimum effective dimension for the implantation openings if optimal doping levels are desired to be obtained with reasonable implantation doses.
As an example, FIG. 2 shows diffusion profiles resulting from the implantation of arsenic through openings of various widths in implantation masks. Direction x is parallel to the surface of a substrate, and the profile is studied at a depth at which the concentration maximum appears. These profiles correspond to a bidimensional simulation. The results are worse in practice since the third dimension would have to be taken into account. In the various cases shown, the implantation dose was 4.1015 at./cm2. In FIG. 2, the ordinates are in atoms/cm3 and the abscissas in micrometers.
For curve 11, the mask opening width is 0.4 xcexcm; it can be seen that the maximum concentration at the center of the buried layer is 1019 atoms/cm3 and the width at mid-height is 1.6 xcexcm.
For curve 12, the mask opening width is 1.4 xcexcm; it can be seen that the maximum concentration at the center of the buried layer is 2.1019 atoms/cm3 and the width at mid-height is 2.6 xcexcm.
For curve 13, the mask opening width is 3.4 xcexcm; it can be seen that the maximum concentration at the center of the buried layer is 2.6. 1019 atoms/cm3 and the width at mid-height is 4 xcexcm.
For curve 14, the mask opening width is 7.4 xcexcm; it can be seen that the maximum concentration at the center of the buried layer is 2.6. 109 atoms/cm3 and the width at mid-height is 8 xcexcm.
Thus, it can be seen that, in these conditions, the maximum doping aimed at at the center of the buried layer is only obtained for a mask having an opening greater than 3.4 xcexcm. For smaller masks, due to diffusions, the maximum concentration decreases. Further, even for a mask of minimum dimension (0.4 xcexcm), due to the lateral diffusion, the resulting curve extends over a width greater than 2 xcexcm. A significant guard distance must thus be provided between two distinct buried layers, for example a distance greater than 1.5 xcexcm.
U.S. Pat. No. 4,862,240 provides for separating two adjoining buried layers of opposite types by a trench. A drawback of this method is that the trench must be wide enough to eliminate the area wherein the dopant of opposite types coexists.
The present invention aims at overcoming the various disadvantages discussed hereabove and especially at enabling formation of buried layers close to one another and having a high maximum doping level.
To achieve this and other objects, the present invention provides a method of forming separate buried layers close to one another in a semiconductor component, including the steps of forming, by implantation, doped areas in a semiconductor substrate; performing an anneal just sufficient to eliminate crystal defects resulting from the implantation; depositing an epitaxial layer; digging trenches delimiting each implanted region; and annealing the buried layers, the lateral diffusion of which is blocked by the trenches, which are deeper than the downward extension of the diffusions resulting from the implantations.
According to an embodiment of the present invention, the guard between the trench-forming mask and the implantation-forming mask is on the order of 0.2 xcexcm.
According to an embodiment of the present invention, the first anneal is performed at a temperature on the order of 900xc2x0 C. for a duration shorter than one hour.
According to an embodiment of the present invention, the second anneal is performed at a temperature greater than 1000xc2x0 C. for a duration greater than 20 minutes.
According to an embodiment of the present invention, the implantations are performed at a dose between 1014 and 1016 atoms/cm2.
According to an embodiment of the present invention, at least one of the implanted areas is formed above a region previously implanted of the opposite conductivity type and previously annealed.
According to an embodiment of the present invention, the previously-implanted region diffuses sufficiently to extend under trenches and join buried layers formed on the other side of these trenches.
The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.